The invention relates to converting pictorial information into electrical signals and to converting electrical signals into pictorial information. More specifically, the invention relates to obtaining Fourier transform representations of pictorial information, and for converting such Fourier transform representations into the corresponding pictorial information. Still more specifically, the invention relates to directly converting between optical images and the Fourier transform representations of the images.
Electronic processing of pictorial information is an active field, and there are many devices for converting between pictorial information and electrical representations thereof. Such prior art devices commonly require arrays of small photosensitive elements. The individual elements of an array are sensed for changes in a photosensitive parameter when an optical image is incident on the array. This is explicitly the case in devices such as photodiode mosaics, and is implicitly the case with devices such as the Vidicon tube, where only a small region of the photosensitive surface contributes at any one time to the video signal derived from the tube. In such prior art devices, the instantaneous value of the derived electrical signal generally represents the light intensity of a particular portion of an image. Such electrical signals may be later processed, such as by analog or digital computers, into a Fourier transform representation of the signals and hence of the image represented by the signals. The Fourier transform representation is desirable, because it allows for more efficient and more versatile electronic processing of images, such as for improving image resolution, removing noise, providing electronic zoom operations, motion and speed detection, pattern recognitions, band width compression, etc.
The advantages associated with the use of Fourier transform representations of pictorial information have led to many devices for obtaining such representations. For example, there are programs for utilizing general purpose digital computers to obtain the Fourier transform representation of electrical signals, and there is a class of special purpose machines called Fast Fourier Transform Computers. Additionally, there is a laser technique for optically obtaining the Fourier transform of laser images. This laser technique is based on the observation that a planar density pattern of coherent light gives rise to its Fourier transform when the pattern is placed in the front focal plane of a lens and the result is observed in the back focal plane (see, for example, Poppelbaum, Computer Hardware theory, McMillan, 1972, pages 626 et seq.). It is emphasized that this laser technique is limited to using coherent light, and cannot be extended to conventional pictorial information which, of course, is a non-coherent and polychromatic optical image.
Because of the desirability of having Fourier transform representations of pictorial information, there is a need to obtain such representations simply and efficiently.
It is known that there are relationships between mechanical deformations of certain materials, optical images incident on the materials and electrical signals associated with the materials. One example of a device utilizing such relationships is disclosed in U.S. Pat. No. 3,202,824 issued to Yando in 1965. The patent relates to a pick-up device employing a photoconductive layer in which a light pattern projected on the layer is transformed into a series of output voltage pulses. These output pulses are produced by propagating an elastic wave accompanied by an electric field along the surface of the photoconductive layer. These output pulses give some information on the relative one-dimensional distribution of light and dark areas of the image, but provide no information about the specific light distribution of the light pattern. The pick-up device does not relate to deriving Fourier transform representations of images. Another prior art device of this type is disclosed in U.S. Pat. No. 3,412,269 issued to Crittenden, Jr. in 1968. The patent discloses a transducer translating electromagnetic wave energy to ultrasonic wave energy. The device includes a slab of cadmium sulfide which is exposed to light of a specific wave length such that alternate dark and light bands are established along the acoustic propagation axis of the cadmium sulfide. The dark and light bands are regions of high and low electrical impedance respectively. The disclosed device does not relate to conversions between optical images and Fourier series or transform representations thereof. Still another prior art device of the type is disclosed in U.S. Pat. No. 3,649,855 issued to Auld in 1972. The disclosed device relates to modulating the conversion of acoustical to electrical energy by varying a light beam illuminating the converting material. Again, the disclosed device does not relate to the conversion between optical images and Fourier transform representations thereof. In fact, applicants know of no prior art technique for directly obtaining electrical signals which are spacial Fourier transform representations of optical images.